The present invention relates to current limiters for transistorized power amplifiers and, more particularly, to current limiters for switching servo amplifiers typically designed to provide .+-.75 V at .+-.10 A to drive DC servo motors.
Transconductance amplifiers are universally used in switching servo amplifiers. The transconductance amplifier acts to force motor current to be directly proportional to input voltage. To protect both the motor and the amplifier, the input voltage is limited to some maximum value. One simple way of limiting the input voltage is by means of two back-to-back series-connected zener diodes connected between the amplifier input and ground. This limits the input voltage to .+-.V.sub.Z, V.sub.Z being the zener breakdown voltage (neglecting forward diode voltage drops). In this manner, the current is also limited, the current being proportional to the input voltage. While this is one possible method of current limiting, it is undesirable because it does not differentiate between current peaks which occur closely spaced in time and those which occur widely spaced in time.
To obtain rapid motor acceleration and maximum motor performance it is often desirable to allow relatively high peak currents for short time periods. DC motors having low inductance (e.g., printed circuit) armatures can accept and respond to large current pulses of very short duration. Current limiters which do not permit high peak currents for relatively short periods, therefore, effectively degrade motor performance. Conversely, current limiters which permit high current peaks regardless of their frequency of occurrence also permit excessive continuous currents which could damage or destroy both the amplifier and the motor.
The present invention overcomes these deficiencies by independently, adjustably limiting both the peak and average current in a switching servo amplifier. The amplifier current is monitored and, when the average current is low, large peak currents are passed to the motor. When the average current is high, however, the large current peaks are limited. In all cases, however, the maximum peak current is limited.